Starting off, mobility is highly important for the Jr. due to its short range. Swim Speed or Run Speed, and I highly recommend Ink Resistance Up shoes.
Damage up is nice in the event you notice a good portion of players are running Defense Up, but don't invest heavily - shooters can't go under their minimum shots to splat. Two or three secondaries should be well enough for contesting defensive builds.
All the Special Up abilities are great for the Jr. as Bubbler is extremely potent. Invulnerability can turn games around easy.
Of course, experiment with what else feels good. You may like just max movement with one damage up sub. If you're comfortable with it, then roll with it; you'll do best with something you're comfortable with as opposed to a "cookie cutter" build.
Also, I don't mean use a roller when I say "roll with it".